The invention relates to a method for determining a rate at which boiler tubes that are exposed to combustion products are corroding and taking steps to reduce the corrosion rate.
For many years electricity has been produced using boilers or furnaces which generate steam that drives a turbine. Many of the furnaces used to produce electricity have a series of tubes that run along or form the inside walls of the furnace. One surface of the tubes faces the combustion chamber and is heated. The tubes are usually made from iron containing metal alloys. During operation of the furnace an iron oxide film forms on the fire side surface of the tubes. Ash particles and slag also accumulate on top of the iron oxide film. That slag is a solution or mixture of iron and silicon oxides which is commonly identified as FexOySiO2. Other chemicals, particularly calcium may also be present in the slag. Depending upon the relative amounts of calcium, iron and silicon present in the slag, the slag will be either liquid or solid at operating temperatures within the furnace. When the ash is liquid, it is generally referred as fused ash, vitrified ash, or most commonly as slag.
Until recent years furnace boiler tubes corroded slowly and had a service life of 20 to 30 years. However, the introduction of low NOx burners has increased the rate of boiler tube corrosion which can reduce their life expectancy to only 1 to 2 years. The result is that not only do tubes have to be replaced at an expense of $2 to $5 million dollars per boiler (for two complete sidewall panel replacements), but the corrosion problem has also resulted in the need to improve coal quality, sometimes doubling the cost of coal. Consequently, there is a need for a method that will reduce corrosion of furnace boiler tube walls in furnaces fired to low NOx emissions. The water inside boiler tubes is at a high pressure, typically from 2000 to about 5000 psi. Consequently, the tubes could fail if their walls become too thin as a result of corrosion. For this reason, the industry has periodically measured the thickness of the walls of its boiler tubes using sonic measuring techniques and other methods. When these measurements indicate that the walls are becoming too thin, the boiler tubes are replaced. While the industry has been able to determine corrosion rates from periodic measurements of wall thickness, corrosion rates determined in this way are of little use in efforts to control corrosion.
The corrosion of furnace wall tubes involves several mechanisms. First, removal of the protective oxide film allows further oxidation. Second, if the oxide film is not present the iron surface is attacked and pitted by condensed phase chlorides which may be present. A third mechanism occurs when wet slag runs across the surface of the film. As that happens, iron from the tube goes into the slag solution which contains low fusion calcium-iron-silicate eutectics that are formed in the liquid slag under reducing conditions in the furnace. Reduced sulfur in the form of S, H2S, FeS or FeS2 can react with the oxygen of the tube scale depriving the tube metal of its protective layer. If one understood what caused each of the mechanisms to occur and could detect when they are occurring, then steps could be taken to prevent corrosion. Yet, prior to the present invention the art has not done this.
Within the past fifteen years corrosion engineers have developed probes and methods which can monitor corrosion rates in real time as corrosion is occurring in a variety of equipment. These probes and methods are based upon a recognition that corrosion is an electrochemical process during which electrochemical noise is generated. Electrochemical noise is a generic term used to describe low amplitude, low frequency random fluctuations of current and potential observed in electrochemical systems. Thus, by placing electrodes in the corrosive environment, one can measure the electrochemical noise that is present. Hladky in U.S. Pat. No. 4,575,678 discloses that measurements of electrochemical noise can be used to calculate a rate at which corrosion is occurring. He further discloses an apparatus for measuring corrosion that is occurring in a variety of liquid containing apparatus such as pipes, storage tanks, heat exchangers, pumps and valves. Eden et al. discloses a corrosion monitoring apparatus in U.S. Pat. No. 5,139,627 which also relies upon measurements of electrochemical noise. This apparatus has been commercialized by Integrity Solutions of Aberdeen, Scotland, and is being sold under the name MENTOR CORROSION SURVEILLANCE system. These devices have been used to measure corrosion in storage tanks and pipes. However, the art has not realized that they could be used in furnaces where temperatures exceed 2000xc2x0 F. and where corrosion occurs because of chloride reactions and metal oxidation, sulfation, and reduction reactions within the wet slag.
We provide a method for monitoring corrosion of furnace boiler tubes by measuring electrochemical noise occurring at the surface of the tubes while that surface is exposed to combustion products. We further provide a method for controlling that corrosion. A probe is provided for measuring electrochemical noise. The probe is connected to a corrosion monitor having a computer and software which determines a corrosion rate from the measured electrochemical noise. That rate is compared to a standard to determine if the rate is within acceptable limits. If not, the operator of the furnace is notified and changes are made to the amount of air or fuel being provided to one or more burners.
Other objects and advantages of the invention will become apparent from a description of certain preferred embodiments shown in the drawings.